1. Field of the Invention
The present invention relates generally to an ultrasonic bonding (or welding) apparatus, and more particularly to an ultrasonic bonding apparatus configured to fill an underfill in a space between a chip (electronic component) and a substrate, and to bond the chip and the substrate with each other through the underfill. The present invention is suitable, for example, for an ultrasonic bonding apparatus configured to mount a bare chip (flip-chip: FC).
2. Description of the Related Art
Flip-chip mounting is a known technology alternative to wire bonding, which electrically connects a chip to a substrate via an array of bumps. The underfill is filled in a space between the chip and the substrate so as to mitigate an external stress and to improve the reliability of the connection. Recently, as a chip size reduces in order of a ball grid array (“BGA”), a chip size package (“CSP”), and the FC, an interval between the chip and the substrate becomes smaller. Thus, filling the underfill after mounting (this process will be referred to as “post-filling” of the underfill hereinafter) becomes difficult, and filling the underfill before mounting is necessary (this process will be referred to as “pre-filling” of the underfill hereinafter).
The ultrasonic bonding apparatus is mounted with the chip on a (tool) surface of an ultrasonic head on the substrate side, and bonds the chip onto the substrate by pressing the chip upon the substrate while applying the ultrasonic wave.
Prior art include, for example, Japanese Patent Applications, Publication Nos. 08-195063, 2001-30465, 56-3447, 05-345411, 2002-53110, 60-234282, 2002-313837, 02-260552, 2004-342847, 2004-207294, 2005-302750, 2005-72033, and 08-264584, Japanese Utility Model Publication No. 06-38828, Japanese Utility Model Registration No. 2,562,897, and Japanese Patent No. 2,987,386.
In the pre-filling, the chip is pressed and bonded while the underfill is being placed on the substrate. Therefore, the underfill under the chip is forced out in the press time, and adheres to and contaminates the tool surface. As the head's temperature becomes higher, the underfill that has adhered to the tool surface cures to lower the parallelism of the tool surface, and the transmissibility of the ultrasonic vibration deteriorates. As a result, the parallelism between the chip and the substrate degenerates, and the bonding quality becomes worse.
Since the ultrasonic bonding apparatus polishes the tool surface so as to maintain the flatness of the tool surface, the post-cured underfill can be eliminated through polishing. However, the pre-cured underfill cannot be removed by using the polishing machine because the pre-cured underfill clogs a polishing grindstone. In this respect, there is proposed a method for applying the ultrasonic wave while a film is held between the tool surface and the chip at the mounting time. However, this method can cause a transmission of the ultrasonic vibration to be insufficient, and a slide occurs on both surfaces of the inclusion (film) and the bonding quality becomes unstable.